The present invention relates to an up-link power control system in an earth station in a satellite communications, in particular, relates to a system in which a signal power received at a satellite is constant even when heavy rain attenuates an up-link power from an earth station to the satellite, by increasing the transmission power of the earth station.
Conventionally, the radio frequency band used in an international satellite communication system is 6/4 GHz, and 1.6/1.5 GHz in an international maritime satellite communication system. An attenuation caused by rainfall in those frequency bands is not very large, except for a rare heavy rainfall condition.
It is a tendency that a higher frequency band is used so as to cope with a rapid increase of traffic, and incorporate small earth stations with small antennas in satellite communication systems. For instance, a Ku band (14/11 GHz), and/or a Ka band (30/20 GHz) is to be used. The rain attenuation in those high frequency bands is considerably larger, and a transmission power control for compensating the rain attenuation is essentially necessary.
A transmission power control is carried out on the total transmission power in an up-link from an earth station so that a satellite receives signals with constant power from the earth station. As for a down-link from the satellite to the earth station, a link design in the satellite communication systems is carried out so that a satisfactory transmission quality is obtained even when the down-link is subject to rain attenuation. Therefore, the transmission power control is not usually carried out totally compensate attenuations both in the up- and the down-links. Controlling only a power in the up-link transmitted from the earth station is called an up-link power control.
It should be noted that the satellite is a mere repeater of signals, but has no facility to measure the power transmitted from each earth station. Therefore, when an earth station controls its transmission power, the earth station has no information about the up-link attenuation, so, an accurate up-link power control is impossible.
In order to solve this problem, two solutions has been proposed.
(1) The correlativity between the up- and down-link attenuations is applied, and the up-link attenuation is presumed, using the attenuation measured in the down-link.
(2) The up-link attenuation is calculated by subtracting the down-link attenuation from a sum of up- and down-link attenuations.
Problems of those two methods are analyzed in the following.
FIG. 1 shows a block diagram of a satellite communications sytem which carries out the above method (1).
A beacon signal transmitted from the satellite 50 is received at the beacon receiver 51, which detects the level of the received beacon signal, and compares the level with a nominal received level to obtain a down-link attenuation. The up-link attenuation estimation circuit 52 presumes an up-link attenuation, based upon the measured down-link attenuation. The transmission power from the transmitter 53 is determined, based upon the value presumed by the estimation circuit 52. A down-link attenuation measured by the beacon receiver 51 does not directly represent the up-link attenuation, because the up-link frequency differs from the down-link frequency. Therefore, the up-link attenuation should be presumed using not only the measured down-link attenuation, but also a correlativity between the up- and down-link attenuations derived from past measurements and experiences.
However, a relationship between the rain attenuation and the frequency bands varies, depending upon the rainfall conditions, and/or the locations of the earth stations, and so, an uniform relationship is not applicable to all the earth stations. Thus, it has been impossible to estimate the up-link attenuation accurately. Furthermore, the method (1) has a disadvantage that the beacon signal generator 49 on-board the satellite is generally unstable, and so, the transmission power of the beacon signal itself fluctuates.
FIG. 2 shows a block diagram for carrying out the method (2). In this method, an earth station transmits a pilot signal generated in the pilot oscillator 54 to a satellite 50. The pilot receiver 56 receives the pilot signal which is relayed by the satellite 50, and measures a power level of the received pilot signal. Simultaneously, the beacon receiver 55 measures a level of a received beacon signal. The power difference detector 57 detects a difference between the levels of the received beacon and pilot signals. The detected difference is considered as an up-link attenuation. The transmission power from the transmitter 53 is controlled based upon the calculated up-link attenuation. This method has an advantage that a correlativity between the up- and down-link attenuations is not used, and so, no error caused by the presumption occurs. However, it still has a problem of fluctuation of the beacon signal generated on-board the satellite. Furthermore, it has a disadvantage because all earth stations have to transmit pilot signals to the satellite. This results in an unefficient utilization of frequency bands.